This invention is related to a method for isolating palynological material from rock samples. More particularly, the invention is concerned with a method useful for isolating pollen and spores in a pressurized reaction cell which provides for the removal of all liquids from the cell without significant loss of sample, followed by centrifugation and zinc bromide separation steps.
Palynological analysis has become an important part of petroleum exploration over the last two decades. Samples of palynological material are used to age-date the levels of rock in underground formations. Fossil spores and pollen have become important as keys for correlation and progressive evolution through time of rock sequences. Particular species of plants and microorganisms may serve as good indices of a certain interval of geoological time by virtue of having arisen by evolution early in the geological interval and having disappeared through extinction at the end of the geological interval.
The isolation of spores and pollen from the mineral matrix of rocks and outcrops is a very time consuming and labor intensive process. Current methods for palynological isolation take two to three weeks to perform as well as considerable attention by a laboratory technician.
A good number of the process steps involved in palynological isolation are the same or similar to the process steps involved in the isolation of kerogen from rock. Kerogen is a solid form of organic matter found in sedimentary rock that is insoluble in water, non-oxidizing acids and bases, and the usual organic solvents. It thermally degrades in a predictable manner by releasing hydrocarbons and condensing the solid organic structure. Because of its immobility, it can provide direct historical evidence of geological conditions within a stratigraphic sequence.
Kerogen, the precursor to oil and gas, must be isolated in such a way that the isolated fraction is as representative as possible of in situ kerogen. Analysis requires the recovery of a sufficient amount of kerogen sample without chemical alteration. A principle objective is to keep the morphology of the kerogen intact and to recover identifiable organic debris such as palynological material.
The isolation of kerogen and palynological material is a complicated chemical process that involves the use of strong acids and bases that dissolve the rock matrix without modifying the desired products. The rock sample is first finely ground to pea-sized particles approximately 1-2 mm in diameter in order to facilitate reaction with the reagents. The methods now used for kerogen isolation employ the dissolution of silicates by hydrofluoric acid and the dissolution of sulfides, sulphates, carbonates, oxides and hydroxides by hydrochloric acid. The reactions are normally carried out below 70.degree. C., a temperature sufficient to dissolve carbonates, but inadequate to promote oxidation and degradation of the organic matter. For a general discussion of the methods of kerogen and palynological material isolation and reagents employed, please see Durand, B., Editor, Kerogen, Graham & Trotman Ltd, London (1980), especially Chapter 2, "Procedures for Kerogen Isolation" by Durand, B. and Nicaise, G., p. 35-52, and Chapter 3, "Les Kerogenes Vus Au Microscope" by Combaz, A., p. 58-60.
Generally the acid and base reactions are carried out in open plastic beakers placed in a steam bath to raise reaction temperature. After reaction, the aqueous liquids are removed from the beakers by decanting. The reaction steps are repeated until isolation is judged to be relatively complete. This usually takes anywhere from two to four weeks depending on the quality requirements of subsequent analyses and the type of rock being dissolved.
The process of decanting as well as the length of the procedure leaves much to be desired. A disadvantage to current isolation techniques is that a percentage of the kerogen and palynological material is frequently lost during decanting. Second, since aqueous liquids are never completely removed from the beakers, solvated metal and silicate ions are available as reactants to form fluoride precipitates. Once precipitated, these neoformed fluorides are virtually impossible to dissolve and remove without damaging the remaining kerogen. Third, beakers permit exposure to oxygen which creates undesirable oxidation products.
Other difficulties exist with current isolation procedures. Dangers to the workers who perform isolations in the standard open beaker method include exposure to hazardous highly concentrated hydrofluoric and hydrochloric acids and bases such as ammonium hydroxide, all of which must be added and decanted manually. Use of such chemicals requires not only protective personal equipment but also engineering controls such as hoods and other vacuum equipment.